Experience has shown that haemocyanin, a protein derived from keyhole limpets, is particularly effective for this purpose, so that was the one the team chose.

To find out whether older generations could indeed pass along immunity, the researchers injected 60 urban pigeons with haemocyanin—a protein that transports oxygen in some invertebrates, including keyhole limpets.

The birds injected with haemocyanin responded predictably: Before long, they developed antibodies that attacked the foreign proteins.

Soon after, a second generation of chicks hatched, and the researchers injected the offspring with haemocyanin.

All members of this third and final generation were injected with haemocyanin.